feat(queue): add queue monitoring dashboard and pending banner on files page

- Add /api/queue/stats endpoint with Redis queue lengths, Celery worker
  inspection, and DB processing summaries
- Add /api/queue/pending-count lightweight endpoint for the files page banner
- Add /admin/queue admin-only view with auto-refreshing queue dashboard
- Add queue pending banner on /files page showing queued/processing count
- Add Queue Monitor link to admin dropdown in navigation (desktop + mobile)
- Add comprehensive tests for all new endpoints and views

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/base.html

@@ -95,6 +95,9 @@
                   <a href="/admin/files" class="flex items-center px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
                     <i class="fas fa-folder-open w-4 mr-2 text-gray-500"></i> File Manager
                   </a>
+                  <a href="/admin/queue" class="flex items-center px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
+                    <i class="fas fa-stream w-4 mr-2 text-blue-500"></i> Queue Monitor
+                  </a>
                 </div>
               </div>
             </div>
@@ -168,6 +171,9 @@
                   <a href="/admin/files" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">
                     <i class="fas fa-folder-open mr-2 text-gray-400"></i> File Manager
                   </a>
+                  <a href="/admin/queue" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">
+                    <i class="fas fa-stream mr-2 text-blue-400"></i> Queue Monitor
+                  </a>
                 </div>
               </div>